CHERBOURG, France  A heavily guarded convoy of vehicles believed to be transporting U.S. weapons-grade plutonium left a plant in northern France on Thursday for a recycling factory 1,000 km (660 miles) southeast.

French state-owned nuclear energy firm Areva, whose Cogema unit will recycle the plutonium into nuclear fuel, declined to confirm the content of the convoy that witnesses saw leave the La Hague plant in the early hours of the morning.

Environmental activists are worried about the safety of the shipment, which arrived in the port of Cherbourg on Wednesday after a more than two-week journey from Charleston, South Carolina, in the United States. They fear it is vulnerable to terrorist attack.

"This is a high-risk strategy being played by the nuclear industry, with the lives of millions of people," said Shaun Burnie of Greenpeace International.

A convoy of several dozen trucks, cars, and buses left the La Hague plant at around 4.30 a.m. Police were guarding all bridges on the convoy's route to the Cadarache plant in southeastern France, where the plutonium will be recycled into nuclear fuel.

This will then be shipped back to the United States for use in an electricity-generating reactor. It is part of the U.S. Department of Energy's program to turn plutonium from "excess" nuclear warheads into mixed-oxide (MOX) plutonium-uranium enriched fuel.

Greenpeace says the shipment is of 140 kg (308 pounds) of plutonium. A spokesman for the U.S. Security Administration said the amount being transported is 125 kg.

The delivery is part of a postCold War agreement between the United States and Russia to get rid of plutonium from excess nuclear warheads.

"The plutonium ... is shipped in casks that comply with the regulation of the International Atomic Energy Agency (IAEA). Its transport is the object of the strongest safety and security measures," Areva said in a statement.

But activists, who expect the cargo to reach Cadarache during the night or on Friday, say the transport is irresponsible, and they have called for a rally close to the southeastern factory later on Thursday.

"Independent expert analysis presented by Greenpeace to the French government earlier this year exposed the potential scale and severity of an accident or terrorist attack on plutonium transports," Greenpeace's Burnie said.

On Wednesday, protesters watched as the boat docked but did not interfere when the plutonium was loaded into a truck to be transported to the La Hague peninsula. A French court ruling has barred protesters from going within 100 meters of the shipment.

Activists on Tuesday bolted a heavy truck to the road leading to La Hague and chained themselves to the vehicle to try to stop the delivery. Police used chain cutters to cut free the protesters and later removed the truck.

Under Tuesday's court ruling, any protester who goes within 100 meters of the shipment faces a 75,000 euro (US$92,230) fine.